«D1-202 CIGRE 2012 21, rue d’Artois, F-75008 PARIS : //A procedure for space charge measurements in full-size HVDC extruded ...»
D1-202 CIGRE 2012
21, rue d’Artois, F-75008 PARIS
http : //www.cigre.org
A procedure for space charge measurements in full-size HVDC extruded cables
in order to access the electric field in the insulation wall
M. MARZINOTTO G. MAZZANTI
Terna S.p.A. Dept. Electrical Engineering – Univ. of Bologna
This paper proposes a procedure for the measurement of space charges in full-size HVDC extruded cables, that accounts for the experimental practices of such kind of measurements in terms of poling time, depolarization time, heating and cooling of specimens. The aim is to access the electric field in the insulation wall under different test conditions. Such procedure to be used in prequalification tests can give the fingerprint of the tested cables and the results could be useful in order to skip the prequalification tests on new cables manufactured with the same insulation and semiconductive compounds, but with different cross section geometry. It must be pointed out that there is no universal agreement about limits for space charge measured. However the change in the relevant electric field with respect to the Laplacian field seems a clear indication of the attitude of an insulation to give rise to field enhancement, and thus to behave unsatisfactorily in service. In any case, the aim of the procedure is not giving maximum acceptable space charge limits, but rather evaluating the electric field profile during the prequalification tests in full-size HVDC extruded cables.
KEYWORDS Space charges, Poissonian electric field, Laplacian electric field, HVDC extruded cables, prequalification tests, poling time, depoling time.
1. INTRODUCTION HVDC cables are becoming more and more popular all over the world. In the recent years, due to research and development about extruded insulation for DC application, extruded type cables have started out on both land and submarine HVDC interties. A critical problem for extruded HVDC cables is the space charge build-up under the constant DC voltage, that strongly affects the Poissonian electric field distribution within the insulation wall. Literature papers claim that in well designed HVDC cables the space charge effect on the field should be moderate . Anyway, other papers  encourage the measurement of space charge on full-size cables, because there is no certainty that the field distribution in small-size samples is the same as in power cables, particularly under temperature gradients.
The measurement of space charge on full-size cables is not an easy task, due to inherent experimental difficulties . However, since the 1990s space charge measurements on full-size cables have been satisfactorily performed [3-7]. Furthermore, the measurement of space charge is essential for the evaluation of the real “Poissonian” electric field in the insulation wall of HVDC cables [2, 3]. In AC cables the evaluation of the electric field in the insulation wall is fundamental for the prequalification tests , since one of the requirements to be matched for skipping the prequalification test on a new cable design is that the electric field in the insulation wall shall not be higher than that of a previously prequalified cable. Today the scientific community is interested in extending similar rules also to DC cables and the only way in order to access to the actual electric field profile in the insulation wall on a DC cable is through a space charge measurement. In addition, the electric field profile in the insulation wall of DC cables varies with both load and environmental laying conditions, as well as with ageing condition. Thus, prequalification tests should be considered an excellent opportunity for the assessment of the electric field in the insulation wall, but at the time being a standard procedure is not universally agreed and the problem has never been approached, at least as to the best knowledge of the Authors of this paper.
In this paper a procedure for space charge measurements in full-size HVDC extruded cables is proposed. This proposal was presented for the first time to the IEEE community at the CEIDP 2011 , and is illustrated here for the second time to the CIGRE community. The procedure, that accounts for the experimental practices of such kind of measurements in terms of poling time, depolarization time, heating and cooling of specimens, has the aim to access the electric field in the insulation wall under different test conditions and is intended to be used in prequalification tests. In this way, the “space charge fingerprint” of the tested cable can hopefully be achieved and the results could be useful in order to skip the prequalification tests on new cables manufactured with the same insulation and semiconductive compounds, but with different cross section geometry.
It must be pointed out that there is no universal agreement about limits for space charge measured , however the change in the relevant electric field with respect to the Laplacian field seems a clear indication of the attitude of an insulation to give rise to field enhancement, and thus to behave unsatisfactorily under service. In any case, the aim of this paper is not to give maximum acceptable space charge limits, but rather - as mentioned above - developing a test procedure to be used to evaluate the electric field profile during the prequalification tests in full-size HVDC extruded cables.
2. MEASURING SPACE CHARGE IN FULL-SIZE DC EXTRUDED CABLES: THELITERATURE
As pointed out above, the measurement of space charge in DC extruded cables is fundamental for the development of such cables, since the field distribution under DC voltage is strongly affected by space charges and determined by many factors, such as carrier mobility, carrier density, and trap density, that cannot be estimated from basic relationships and data of the material . The first attempt to measure the space charge distribution in a small-size coaxial cable by the Pulsed Electro-Acoustic (PEA) method1 was carried out by Fukunaga et al.  via a coaxial outer electrode in close contact with the cable. A piezoelectric transducer of Poly-Vinyli-Dene Fluoride (PVDF) film was tightly For details about PEA method, refer e.g. to [2, 3, 10].
wound around the coaxial shaped electrode in order to prevent acoustic wave reflection at the interface between the electrode and the transducer [3, 11].
When dealing with large cross-section HV cables, several difficulties arise. Large HV terminations have to be mounted, that require an adequate cable length. Moreover, if the PEA pulse voltage is applied at the termination, it travels through the cable and the waveform of the voltage at the measuring point would not be a proper single pulse, due to the distortion and/or reflection of the injected pulse – since in this case the characteristic impedance of the cable has to be considered.
Hozumi et al.  designed an experimental system capable of solving these problems, in which the pulsed voltage is applied between the measuring point at the centre of an exposed semiconducting portion and ground. Typically, the height of voltage pulses is a few kV and the width is some tens ns.
If the characteristic impedance of the cable is much lower than the impedance of the capacitance at the measuring point in the frequency range of the voltage pulses most of the voltage pulses is applied at that point. The acoustic wave is measured using a PVDF film that is typically around 100 μm in thickness. The signal is amplified inside a shielded box and transferred by an optical fiber [3-5, 12].
A further problem in space charge measurement on full size cables is the considerable thickness of their insulation, that can be as high as ~20 mm or more. As a consequence, pressure waves coming from the vicinity of the inner electrode (i.e. the conductor) are highly attenuated and distorted. This leads to calibration problems, that can however be overcome, as stated in .
Skipping further details about the PEA measurement set-up for a full-size cable, that are outside the goal of this paper, it can be said that at least two space charge measurement campaigns on full sized
cables were performed by means of this set-up, namely:
1. on 250 kV XLPE insulated cables, 12 m long including terminations, with 800 mm2 crosssection conductor and 20 mm thick insulation, poled at 500 kV (for a maximum of 3 h on each polarity) under a severe temperature gradient (ranging from 85 °C on conductor to 5-7 °C in open air where the cables were laid; typical pulse voltages were 50 ns wide and 5 kV high [4, 5]);
2. a second one on a 500 kV XLPE insulated cable with 3000 mm2 cross-section conductor and 23 mm thick insulation, poled at 500 kV under temperature gradient (conductor at 90 °C, other details are missing [6, 7]).
Also the Thermal Step Method (TSM) has enabled space charge measurements on cables with thick insulation, as it can be found in some well known literature papers. For instance, space charge measurements by means of the TSM technique were performed on XLPE insulated prototype HVDCcables with 95 mm2 cross-section conductor and 5 mm thick insulation, poled at –150 kV and –230 kV under temperature gradient . Moreover, space charge measurements via the TSM are being planned at present according to private communications between Pierre Mirebeau Head of Nexans R&D and the Authors of this paper.
In addition, also the Pressure Wave Pulse (PWP) method, especially when implemented according to the Laser Induced Pressure Pulse (LIPP) technique, has been considered for space charge measurements on insulation thicker than that of mini-cables (say, up to 3 mm and more) [14-16], but no measurements with this method has never been performed on larger cables as to the best knowledge of the Authors of this paper.
All these evidences confirm that notwithstanding the experimental difficulties the measurement of space charges on full sized cables is feasible and has in fact been performed. In addition, such measurements are strongly fostered by authoritative researchers in the field, as reported in .
3. MEASURING SPACE CHARGE IN FULL-SIZE DC EXTRUDED CABLES: THEPROPOSED PROCEDURE
The space charge measurement procedure proposed here aims at detecting the space charge distribution and the electric field profile within the insulation wall of the full-size cable (i.e. the same type of cable subjected to prequalification and type tests) at the beginning, along and at the end of a certain poling period. Once stabilized, the measured field profile should be compared with the design Laplacian field of the cable and with the field measured at the beginning of the poling time, so that field enhancements potentially harmful for cable insulation in service life can be singled out [17-19].
It must be emphasized that there is no universal agreement about acceptable limits for measured space charge . Nevertheless, a magnification of the relevant electric field with respect to the design Laplacian field denotes the attitude of the insulation to undergo field enhancements over poling time, and thus to behave unsatisfactorily under service. This holds also for comparing the performances of the as-manufactured cable insulation with those of the aged cable.
In this perspective, space charge measurements can also help in understanding if the prequalification tests and/or type tests should be repeated or not in the case of HVDC cables, for which the Laplacian field profile cannot be taken – strictly speaking – as a reference for comparing the electrothermal stress level of new projects with that of previously manufactured cables.
In force of the results reported in the literature about space charge measurements in full-size HVDC extruded cables [3-7], the PEA method should be better chosen. An alternative method that appears to be well-suited for full-size power cables is the TSM. For this reason, in the following the space charge measurement procedures to be followed when using either the PEA or the TSM technique are illustrated in sequence.
Whatever the chosen technique, according to the practical experience a full-size cable length of 10 m at least should be selected to perform the space charge measurements. Each measurement has to be provided in terms of electric field and space charge density profile in the insulation wall. Obviously, if the electric field profiles in the insulation walls of two different cables have to be compared (as in the case of a cable prequalification test already performed vs. a future cable prequalification test), the same space charge measurement method shall be employed for both cables, i.e. either the PEA or the TSM.
A. PEA Method
The proposed test procedure consists of the following steps:
1. The cable shall be heated and the conductor kept at a proper fixed temperature (for example, higher than its rated temperature) for a time sufficient for reaching the thermal equilibrium (e.g. 12 or 24 hours, depending on cable characteristics), and later on maintained at that temperature for the whole test duration.
2. The rated voltage shall be applied between the conductor and the metal sheath (grounded) with positive polarity: the positive voltage poling time, tPV,ON, will start. Immediately after the attainment of the rated voltage, i.e. at tPV,ON=0+, a first volt-on measurement shall be performed, that represents the volt-on reference at the starting of the positive polarity measurements.
3. A sequence of volt-on measurements shall be performed every 20-40 min , until a time ranging from 90 min (1.5 hours)  to 180 min (3 hours) [4-7] has passed. E.g., a possible sequence of measurement times is tPV,ON(1)=40 min, tPV,ON(2)=80 min, tPV,ON(3)=120 min, tPV,ON(4)=160 min.
4. The maximum variation between the electric field profiles achieved at the two last consecutive measurements shall be estimated checking for the stabilization. E.g., in order to avoid excessive test length, stabilization can be assumed as having occurred if such variation does not exceed a value ranging from 5% to 10% [2-7, 11-13, 20-22]; if this does not hold, the sequence at previous point 3 is repeated. According to the previous example, a possible sequence of further measurement times is tPV,ON(5)=200 min, tPV,ON(6)=240 min, tPV,ON(7)= 280 min, tPV,ON(8)=320 min.